After Japanese-American FBI agent Taki Kazuya and Kamen Rider 1 defeat a bat monster, they decide to contact the former riders including Stronger to investigate the appearance of monsters attacking them and civilians. Their investigation reveals the coming of an alien empire out to destroy the Earth, finishing the work that Shocker and other terrorist organizations have failed due to their defeat at the Rider's hands.

Ah, this is like porn for a Showa Rider fan, it's pretty much non-stop action and it even fills in a lot of gaps early on, like how Riderman was blown up and just showed up again in the show, it fills in the gaps here.